As far as I know, they get the serial number off the deck then enter it into a computer which gives out the code. All the details of the unit are recorded at manufacture.
1992 AXII Telstar TX5 Ghia
121kW 2.5L DOHC V6 in a car that weighs 1100kg's and looks like a 4 banger from the outside......SLEEPER!
1988 EA S Pak 3spd CFI.....Soon to be a 250rwkw HSV eating machine.